Erika Saalau Rojas, Laura Jesse, and Mark L. Gleason
Department of Plant Pathology and Microbiology, Iowa State University
BACKGROUND INFORMATION - PART A
Cucurbit Bacterial Wilt
Curcurbit bacterial wilt is a disease that affects economically important crops such as melon, cucumber, and squash.
The bacterium that causes this disease, Erwinia tracheiphila, is transmitted by striped cucumber beetles (Acalymma vittatum) (Figure 1) and survives the winter in the beetles’ digestive systems. Infection occurs when bacteria-infested beetles chew on young cucurbit plants and deposit bacteria-infested frass (feces) onto fresh feeding wounds on leaves and stems. Once E. tracheiphila enters the plant, the bacterium multiplies and plugs the vascular system, causing plants to appear water-stressed and wilted (Figure 2). Infected plants start showing symptoms 1 to 3 weeks after infection. As infection progresses, plant tissues collapse and die before yielding any fruit (Figure 3).
A diagnostic test to determine whether a plant is infected with bacterial wilt can be done in the field. A wilting vine can be cut with a sharp knife. The cut ends are then slowly pulled apart. If the plant is infected, threads of bacterial ‘ooze’ can be seen attached from both ends (Figure 4).
The disease cycle begins when overwintering adult beetles become active in spring. Cucumber beetles are highly attracted to young cucurbit plants and they may infest a cucurbit field less than 24 h after seedling emergence or transplanting. The risk of infection by E. tracheiphila is highest during the spring when beetle populations are high and plants are more susceptible.
Once in a cucurbit field, overwintering beetles feed and mate. Females lay their eggs at the base of cucurbit plants. Cucumber beetle larvae feed on the roots of the plants, pupate and several weeks later emerge as adult beetles. This first generation of beetles can re-acquire the pathogen by feeding from bacterial wilt-infected plants and continue the spread of E. tracheiphila to healthy plants. Up to two or three generations of beetles may be observed in a single season depending on the region. Late in the season, cucumber beetles seek overwintering sites near the field and begin the disease cycle the following spring.
For more information about bacterial wilt, university extension bulletins can be found online.
What is Colony Collapse Disorder (CCD)?
In 2006, an unexplained loss of worker bees in honey bee hives was noticed among beekeepers across the United States. This mysterious disappearance amounted to 30 to 90% of bee colonies – an alarmingly high rate of loss. It was particularly puzzling to scientists that the hives affected showed no obvious signs of disease, and that the dead bodies of the missing bees were nowhere to be found. This phenomenon of missing bees is referred to as Colony Collapse Disorder (CCD).
In the past few years, CCD has received the attention of researchers, industry, media, and the general public because it represents a huge threat to agriculture. Honey bees are the primary pollinators of about 130 different crops in the U.S.A. and it is estimated that bees help produce approximately one third of the food that we eat.
Potential causes of CCD
Several pathogen and parasite infections have been implicated in causing CCD. The microsporidian fungus Nosema ceranea, Varroa mites, and viral infections (i.e. Israeli Acute Paralysis Virus) have been frequently associated with CCD colonies. Some studies have correlated the occurrence of two or more pathogens or parasites with CCD colonies; however, it is unclear to researchers whether these infections cause CCD themselves or weaken colonies so that they are more vulnerable to other sources of stress.
Poor bee hive management practices and environmental stressors have also been pinpointed as possible causes of CCD. Commercial bee hives are often overcrowded and as a consequence bees may become undernourished and stressed. Other factors involved include frequent migration and shipping of bee hives across the country to provide pollination services in high value crops. Bee hives are often rented and may travel thousands of kilometers in a single season, resulting in prolonged disturbance of colonies.
Some researchers consider that habitat changes may also act as stressors. Disturbance in the natural landscape from urban sprawl and intensive agricultural systems could contribute to bee population decline by limiting the availability of food and water for foraging bees.
Among the many suspects in the CCD mystery, insecticide exposure is probably the most controversial topic. Several recent research reports demonstrated that exposure to a specific family of insecticides, called neonicotinoids, could be linked to bee die-offs and CCD. Even at sub lethal doses, neonicotinoids can disorient bees and weaken colonies, possibly making them more prone to CCD. However, scientists have not provided a direct correlation between neonicotinoid exposure and CCD.
Recommendations to growers, beekeepers, and general public
Although fingers have been pointed at numerous causes, scientists seem to agree that CCD is most likely the result of a combination of the above-mentioned factors. However, research is ongoing and over the next few years we can hope for a clearer explanation of CCD. In the meantime, recommendations for beekeepers are to use management practices that enhance honey bee health and improve their habitat.
For the general public and farmers, USDA and EPA recommend limiting the use of pesticides by implementing Integrated Pest Management and to avoid applying pesticides during mid-day.
For more information about research directions and recommendations visit USDA and EPA websites.
BACKGROUND INFORMATION - PART B
Managing cucurbit bacterial wilt
Cucurbit bacterial wilt can be a difficult disease to control. Erwinia tracheiphila, the causal agent, is harbored and spread by cucumber beetles (Figure 5). Unlike other bacterial diseases, cucurbit bacterial wilt is a vascular disease and transmission cannot be stopped by using copper sprays or bactericides. Infected plants usually die in a couple of weeks after showing wilt symptoms. A second challenge in managing this disease is the lack of commercial cucurbit varieties resistant to bacterial wilt. Therefore, control of cucurbit bacterial wilt relies on managing cucumber beetles to prevent infection. Some chemical and cultural practices are listed and explained below:
Contact insecticides (i.e., those that have their effect by coming into direct contact with target insects in the environment or on the outside surfaces of plants) such as carbamates or synthetic pyrethroids can be effective against cucumber beetles but need to be applied every 4 to 5 days when beetle populations are high. Neonicotinoids, a widely used family of systemic insecticides (systemic insecticides enter tissues of crop plants, where they can deter insect feeding or other behaviors), are highly effective and are usually applied at transplant. Soil applications of neonicotinoids can protect young plants for as long as 3 to 5 weeks after application. Cucurbit farmers often use neonicotinoids at the beginning of the season and follow up with weekly or biweekly applications of contact insecticides for the rest of season. For more information about approved products to control cucumber beetles, regional vegetable production guides for commercial growers can be accessed online.
In order to avoid unnecessary insecticide use, regular scouting for cucumber beetles is recommended. Cucurbit fields should be scouted within 48 h after transplanting or seedling emergence. Direct scouting of cucumber beetles should be done in a systematic way at least two to three times per week from the beginning of the growing season until harvest begins. Economic thresholds may vary among geographic regions; however, an economic threshold of one or two beetles per plant (on average) is common. Scouting can save one to two insecticide applications compared to spraying by the calendar without regard to beetle populations. For regional recommendations, university extension guidelines can be found online.
The highest risk of bacterial wilt transmission occurs early in the spring, when high numbers of overwintering beetles become active and locate recently transplanted cucurbit fields. In the Upper Midwest and Midwest regions of the U.S., delaying planting of cucurbit crops until mid- to late June can reduce incidence of bacterial wilt by avoiding contact between young cucurbit seedlings and overwintering populations of cucumber beetles. This strategy is practical for commercial growers who are not concerned about harvesting as early as possible in order to obtain premium market prices.
Row covers are made of breathable, non-woven polypropylene material. They are often used to protect young plants from harsh weather conditions and insect pests. Row covers can act as a physical barrier against cucumber beetles when placed over cucurbit seedlings immediately after transplanting or seedling emergence (Figure 6).
Installation of row covers is expensive and labor-intensive. Row covers are deployed over wire hoops placed every 2 to 3 feet and the edges need to be tightly secured by weighing down or burying into the ground (Figure 7). At anthesis, the start of female bloom (Figure 8), pollination needs to be enabled to avoid harvest delays. Different timing of removal strategies and information about row covers can be found here.
Perimeter trap crops (PTC)
The principle behind PTC relies on using a perimeter of plants that act as a barrier to protect a main crop, called the cash crop, from insect pests such as cucumber beetles. The advantage of using a PTC is that it limits the amount of insecticide to a much smaller area, since most or all insecticide sprays target only the perimeter around the main crop.
In order for a PTC to work, a cucurbit crop that is much more attractive to cucumber beetles than the main crop should be used in border rows surrounding the main crop (Figure 9). The PTC should also be less susceptible to bacterial wilt than the main crop, and preferably should not compete with the main crop. Field research has shown that PTCs are more effective when 1 to 3 rows of the perimeter crop are planted around the main crop. Trap crop planting should be done one to two weeks before planting of the main crop The PTC and the main crop should be scouted on a regular basis, about 2 to 3 times per week; a threshold of 1 beetle/plant triggers insecticide sprays in the border crop or main crop when the risk of bacterial wilt is high.
Several factors should be considered before implementing PTC. Using a PTC means that some percentage of the farm acreage will be dedicated to a crop that is different from the main crop. For example, certain winter squash varieties can be used to protect a main crop of muskmelon, but this arrangement means that farmers will often have to manage two somewhat different sets of diseases and pests, harvest the two crops at different times, and market two different types of produce. For more information about the use of PTC against cucumber beetles and bacterial wilt, please consult the Organic Cucurbit Growing community website.
Bartels, R. 2012. Protecting against bacterial wilt. American Vegetable Grower, October 2012, pp. 26-28.
Egel, D., Foster, R. E., Maynard, E., Weinzierl, R., Babadoost, M., O'Malley, P., Nair, A., Cloyd, R., Rivard, C., Kennelly, M., Hutchinson, B., and Gu, S. 2013. Midwest Vegetable Production Guide for Commercial Growers 2013. 208pp.
Lam, F., and Foster, R. E. 2010. Vegetable Insects -- Monitoring and Decision Making for Cucumber Beetles on Muskmelon. Purdue Extension E-101-W, Purdue University.
An IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky. Cooperative Extension Service. University of Kentucky, Lexington, KY (ID-91).
Sustainable Agriculture Research & Education (SARE). 2012. Optimizing Row Covers and Perimeter Trap Crops for Cucurbit Pest Management. (Annual Project Summary).